Adding Yttrium to SO₄ ^2–/ZrO₂ Catalysts To Improve the Synthesis Performance of Esters for Electrical Insulating Oils

Abstract

Synthetic ester insulating oils exhibit excellent electrical insulating properties, biodegradability, and oxidative stability, rendering them attractive alternatives to conventional mineral- and vegetable-based insulating oils. Typically, synthetic ester insulating oils are prepared via esterification reactions between fatty acids and alcohols using various catalysts. In particular, the solid acid catalyst SO₄ ^2–/ZrO₂ is commonly used in this process. In this study, a small amount of yttrium was substituted into a ZrO₂-catalyst support to form oxygen vacancy structures in the framework. This modification was intended to alter the interactions with SO₄ ^2– ions and improve both the distribution of acid sites and catalytic activity for esterification. Using the sol–gel method, the synthesized catalyst was calcined at 600 °C for 3 h to promote crystallization. The structural and surface properties of the catalyst, as well as its chemical states, thermal stability, electronic structure, and acidity, were investigated using various advanced characterization techniques, including XRD, XPS, and NH₃-TPD. The catalytic performance was evaluated through the esterification of fatty acids (decanoic acid (capric acid) and 2-ethylhexanoic acid) and alcohol at 220 °C. Among the fabricated catalysts, the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst demonstrated the highest conversion of 95.5%. Thus, our developed catalyst could contribute to improving the efficiency of synthetic ester production processes for insulating oil applications.

1. Introduction

Although mineral oils are generally used as transformer insulating oils, they pose significant concerns regarding environmental pollution due to their low biodegradability and potential fire hazards. , These risks have prompted extensive research into eco-friendly alternatives. While vegetable-based ester insulating oils have been partially adopted, their molecular structure containing carbon–carbon double bonds renders them vulnerable to oxidative degradation, which can lead to sludge formation and deterioration over time. Therefore, for the harsh operating conditions of offshore substations, insulating oils with more stringent property requirements are needed.

Offshore substations are located far from the mainland and are exposed to harsh environmental factors, such as seawater, salt spray, high humidity, and strong winds. Therefore, insulating oils used in offshore substations must possess multiple essential properties, including excellent electrical insulating performance, high biodegradability and oxidative stability, superior water and corrosion resistance, a high flash point, and a low pour point. Conventional mineral oils present a significant risk of marine pollution owing to their low biodegradability, prompting the use of vegetable-based esters because they offer better environmental performance. However, their molecular structure contains carbon–carbon double bonds, rendering them vulnerable to oxidative degradation, which can result in sludge formation and deterioration over time. To address these challenges, synthetic ester insulating oils have attracted attention as promising alternatives. Unlike vegetable esters, synthetic esters can be intentionally designed by selecting specific saturated fatty acids and alcohols. This tailored approach allows for the creation of highly stable, saturated molecules that lack the vulnerable double bonds, thereby providing superior oxidative stability and performance characteristics required for demanding applications.

Ester insulating oils are produced via esterification, which is a chemical reaction between fatty acids and alcohols, using catalysts to enhance the reaction efficiency. Esterification reactions are typically accelerated using either homogeneous or heterogeneous catalyst. , While homogeneous catalysts show high reactivity, they are difficult to separate and recycle, leading to wastewater generation. In contrast, heterogeneous catalysts offer significant advantages such as easy separation, reusability, and reduced environmental impact, making the development of highly efficient heterogeneous catalysts a major research focus. Indeed, recent efforts have explored a wide range of materials, from advanced nanomaterials to biomass-derived catalysts, to create more sustainable and efficient systems for esterification. , A wide range of heterogeneous solid acids have been explored for esterification. These include sulfated catalysts (e.g., SO₄ ^2–/ZrO₂, SO₄ ^2–/TiO₂, SO₄ ^2–/SnO₂), − metal oxides (e.g., SnO₂–TiO₂, WO₃), , zeolite (e.g., H-ZSM-5, H–Y), , and heteropoly acid (e.g., H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiW₁₂O₄₀, H₄SiMo₁₂O₄₀). − In recent years, various heterogeneous catalysts have been applied to esterification reactions, including metal oxides, hydrotalcites, porous polymeric materials, heteropolyacids, and MOF-derived materials. Among various heterogeneous solid acids, sulfonated zirconia (SO₄ ^2–/ZrO₂) is particularly notable for its high activity and thermal stability, rendering it a promising candidate for producing synthetic ester insulating oils.

The performance of such a solid acid catalyst in the required esterification reaction is critically dependent on its acidic properties, including the type (Brønsted vs Lewis), density, and strength of the acid sites. Specifically, Brønsted acid sites are known to be highly active for the protonation of the carboxylic acid, a key step in the reaction, while an optimal distribution of acid strength is crucial for achieving high conversion while minimizing unwanted side reactions. While sulfonated zirconia (SO₄ ^2–/ZrO₂) is a promising catalyst, the key research gap remains the development of a strategy to systematically tune these vital acidic properties to maximize its efficiency.

This study addresses this gap by proposing a novel approach: incorporating a small amount of yttrium into the sulfated zirconia catalyst. The key novelty of this work, distinguishing it from previous reports, lies in (i) a modified synthesis route where sulfate is introduced at the Zr­(OH)₄ stage to promote a stronger metal–support interaction, and (ii) the application of this advanced catalyst to a practical synthesis of insulating oil from specific industrial precursors. We hypothesize that yttrium incorporation will create controlled oxygen vacancies, thereby optimizing the acid site strength and distribution to significantly improve esterification reactivity. To validate this, the structural properties, surface chemistry, and acidity of the synthesized catalysts were systematically evaluated to establish a clear structure–activity relationship.

2. Results and Discussion

As shown in Figure , XRD was conducted to investigate the crystallinity of the synthesized catalysts. Figure a shows the diffraction peaks of SO₄ ^2–/ZrO₂ at 2θ = 30.25°, 50.21°, and 60.15°, corresponding to the (101), (112), and (211) planes of the tetragonal phase of ZrO₂, respectively. As shown in Figure b, even after the partial substitution of Y₂O₃ in the catalyst, the tetragonal ZrO₂ crystal structure was retained, indicating that crystallinity was preserved. Conversely, the SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalyst, also with a partial substitution of Y₂O₃, exhibited an amorphous phase of ZrO₂, as shown in Figure c. The lack of peaks corresponding to Y₂O₃ were likely due to the low substitution amount, which was insufficient to induce detectable crystallinity. Thus, Y₂O₃ substitution influences the crystal structure of ZrO₂. In solid oxide electrolytes containing yttria-stabilized zirconia, only the tetragonal ZrO₂ peaks appear in the XRD patterns, even with up to 9 mol % of Y₂O₃. However, excessive Y₂O₃ incorporation suppresses crystal growth, resulting in a loss of crystallinity.

1.

XRD patterns of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

Figure presents the SEM and EDS mapping images of the synthesized catalytic particles, which were approximately several hundred nanometers in size. The major elements of the catalyst, Zr, O, and S, were abundantly distributed across the particle surfaces. The EDS mapping images confirmed the presence of yttrium on the catalyst surface after its minimal addition, and this distribution became more pronounced with increasing yttrium distribution density.

2.

SEM-EDS mapping images of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

Figure shows the N₂ adsorption–desorption isotherms of the catalysts. According to the IUPAC classification, all the catalysts exhibited type III isotherms with H3 hysteresis loops. The synthesized catalysts were nonporous materials, and the observed hysteresis during adsorption and desorption was attributed to bulk pores formed between aggregated particles. Moreover, the amount of N₂ adsorbed on the catalyst surface increased with increasing yttrium content. The specific surface areas of the SO₄ ^2–/ZrO₂, SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts were 31.712, 30.573, and 30.984 m²/g, respectively, as summarized in Table .

3.

BET results of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

1. Specific Surface Areas and Acid Site Densities of the Catalysts.

		Density of acid site
Catalyst	Specific surface area (m2/g)	mmol/g	mmol/m2
SO4 2–/ZrO2	31.712	3.293	0.104
SO4 2–/ZrO2(0.98)-Y2O3(002)	30.573	3.566	0.117
SO4 2–/ZrO2(0.96)-Y2O3(0.04)	30.984	3.052	0.098

The FT-IR spectra of the synthesized catalysts are shown in Figure . An absorption peak attributed to the O–H stretching vibration from physically adsorbed H₂O on the catalyst surface was observed at wavelengths >3000 cm^–1, , while that corresponding to the O–H bending vibration from chemically adsorbed H₂O was observed near 1635 cm^–1 . This assignment is in good agreement with the literature for sulfated zirconia and other metal oxides. For all samples, characteristic peaks corresponding to surface sulfate species were observed. According to the literature, sulfate ions on zirconia typically exist in two main coordination modes: chelating bidentate and bridged bidentate. , Each of these modes gives rise to multiple vibrational modes. The observed peaks at approximately 1203, 1108, and 1056 cm^–1 are a combination of these various vibrational modes from the two primary modes, confirming that SO₄ ^2– was successfully bonded to the catalyst surface. Moreover, the intensity of the absorption peak at approximately 1108 cm^–1 decreased with increasing yttrium content. This result was consistent with the XRD results and suggests that yttrium addition inhibited ZrO₂ crystal growth, thereby reducing ZrO₂ tetragonal structures and SO₄ ^2– bonding. Furthermore, the peaks at approximately 785 and 602 cm^–1 were attributed to Zr–O bonding.

4.

FT-IR spectra of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

Figure shows the UV–Vis spectra of the synthesized catalysts, where absorption in the UV region (<300 nm) was observed. In particular, a strong absorption peak at 283 nm was detected for the SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) particles, and its intensity increased with yttrium content. This absorption is attributed to the charge-transfer on coordinatively unsaturated (i.e., low-coordination) Zr⁴⁺ ions. It is well-established that the absorption for such charge-transfer transitions occurs at a long wavelength for ions with lower coordination numbers. As yttrium content increases the amorphous character of the catalyst, the concentration of these low-coordination sites increases, resulting in a higher intensity for this characteristic absorption peak. This relationship between the number of low-coordination sites and optical signal intensity is a well-established principle. The band gap of the catalysts, determined from the UV–vis spectra, showed a nonmonotonic trend with yttrium content. The band gap first decreased from 3.54 eV for the SO₄ ^2–/ZrO₂ to 3.34 eV for the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02). This initial decrease is attributed to the formation of oxygen vacancies. When Y³⁺ substitutes for Zr⁴⁺, oxygen vacancies are created, which introduce defect-induced energy levels within the original band gap of ZrO₂, effectively reducing the measured optical band gap. This is consistent with our XPS results showing the highest oxygen vacancy concentration for the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst. However, upon increasing the Y₂O₃ content to 4 mol %, the catalyst became amorphous, and the band gap significantly increased to 3.78 eV. This phenomenon is attributed to the quantum confinement effect. The loss of long-range crystalline order results in the formation of extremely small, disordered nanodomains. In such confined nanostructures, the electronic energy levels become more discrete, leading to widening of the band gap. For the SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalyst, this quantum confinement effect, driven by amorphization, becomes the dominant factor, overriding the effect of the defect states.

5.

UV–vis spectra of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

As shown in Figure , XPS was performed to investigate the oxidation states of Zr, S, and Y in the synthesized catalysts and the presence of oxygen vacancies. Figure a shows that the O 1s peak of the SO₄ ^2–/ZrO₂ catalyst was deconvoluted into: the oxygen vacancy (534.25 eV)-2.5 at%, sulfate oxygen (532.4 eV)-53.8 at%, and metal oxide (530.55 eV)-16.4 at%. The Zr 3d peaks at SO₄/ZrO₂ (183.45 eV)-8.8 at% and ZrO₂ (182.8 eV)-4.8 at% corresponded to surface-reacted SO₄ ^2– species, and the S 2p peak was detected at SO₄ (169.33 eV)-14.6 at%. As shown in Figure b, a peak of Y 3d at SO₄/Y₂O₃ (159.33 eV)-0.5 at% was attributed to the small amount of incorporated yttrium for the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst. Additionally, an increase in the O 1s component at the oxygen vacancy (534.25 eV)-4.3 at% was observed. This result supports the decrease in band gap observed in the UV–vis spectrum for the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst. Figure c shows that the Y 3d peak at 159.33 eV increased to 1.1 at% for the SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalyst, while the O 1s component corresponding to the oxygen vacancy at 534.25 eV decreased to 3.2 at%. In Figure d, the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst exhibited the highest oxygen vacancy content (4.3%), and the corresponding area-based atomic percentages are presented in Table and Figures d and e. Also, the at% values determined from the XPS analysis are summarized in Table . Figure d presents variations in the at% of the oxygen vacancy in the O 1s region as a function of yttrium incorporation, while Figure e shows changes in the at% of the metal oxide in O 1s and SO₄ in S 2p with increasing yttrium content. As the yttrium content increased, the bonds corresponding to the ZrO₂ metal oxide decreased, whereas the SO₄ ^2–-related bonds increased. Thus, the exposed catalyst surface was primarily bonded to metal and SO₄ ^2– ions, and adding yttrium decreased the number of oxygen vacancies and Zr–O bonds within the catalyst structure. Sulfated zirconia (SO₄ ^2–/ZrO₂) is a common solid acid catalyst that exhibits high activity in various acid-catalyzed reactions. Incorporating sulfate ions onto the ZrO₂ surface results in the formation of oxygen vacancies, which play a crucial role in enhancing catalytic performance by increasing the number and strength of acid sites. , These vacancies stabilize metastable tetragonal ZrO₂ and suppress crystallite growth, thereby increasing surface area and preserving active phases. Furthermore, oxygen vacancies alter the surface electron distribution and contribute to localized charge deficiency, which facilitates the adsorption and stabilization of reaction intermediates.

6.

XPS spectra of (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04), (d) Y 3d and O 1s vacancy at%, and (e) S 2p and O 1s metal oxide at%.

2. Atomic Analysis from XPS (Atomic %).

	Atomic analysis from XPS (atomic %)
Catalyst	Y 3d (SO4/Y2O3)	O 1s	Vacancy (534.25 eV)	Sulfate (532.4 eV)	Metal Oxide (530.55 eV)	Zr 3d	SO4/ZrO2 (183.45 eV)	ZrO2 (182.8 eV)	S 2p (SO4)
SO4 2–/ZrO2	0	72.7	2.5	53.8	16.4	13.6	8.8	4.8	13.7
SO4 2–/ZrO2(0.98)-Y2O3(0.02)	0.5	73.3	4.3	53.8	15.2	12.5	8.5	4.0	13.7
SO4 2–/ZrO2(0.96)-Y2O3(0.04)	1.1	72.4	3.2	54.5	14.7	12	8.3	3.7	14.6

7.

TGA curves of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

The TGA results of the catalysts are shown in Figure . Two major weight loss steps were observed at approximately 50–200 and 500–700 °C. The first weight loss step (50–200 °C) was attributed to the desorption of physically adsorbed H₂O from the catalyst surface. The second weight loss step (500–700 °C) was ascribed to the decomposition of SO₄ ^2– species bound to the metal oxide, resulting in a weight loss of approximately 30 wt%. In the yttrium-incorporated catalysts, the decomposition temperatures shifted to higher values, indicating improved thermal stability. The maximum decomposition peak temperature increased from 660.13 °C for the SO₄ ^2–/ZrO₂ to a maximum of 683.41 °C for the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst, before slightly decreasing to 680.28 °C for the SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalyst. This trend suggests that while yttrium incorporation strengthens the interaction between the sulfate groups and the zirconia support, excessive content leads to a loss of crystallinity. The slight decrease in stability for the SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalyst can be attributed to its amorphous structure, as observed in the XRD results. The disordered structure may provide slightly less stable anchoring for the sulfate group compared to the optimized, crystalline structure of the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst.

Because the synthesized catalysts functioned as acid catalysts in the esterification reaction, the surface acidity was evaluated using NH₃-TPD (Figure ). Considering the TGA results, the NH₃ desorption experiments were conducted within the temperature range of 50–500 °C. A small desorption peak attributed to weak acid sites was observed at <200 °C, while a strong peak corresponding to medium-strength acid sites appeared between 200 and 450 °C. Additionally, a peak associated with strong acid sites was detected at temperatures >450 °C. Although the weak acid sites were similarly observed across all the catalysts, differences were evident in the medium- and strong-acid regions. Based on the peak areas, the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst adsorbed and desorbed the largest amount of NH₃. This enhanced acidity is directly correlated with the optimized formation of oxygen vacancies at this doping level, as confirmed by our XPS analysis. The incorporation of Y³⁺ creates oxygen vacancies, which contribute to the overall acidity in two ways: (i) they can act as Lewis acid sites themselves, and (ii) they inductively strengthen nearby Brønsted acid sites by withdrawing electron density. This dual role of surface defects and sulfate groups in enhancing acidity is well-established principle for sulfated zirconia system. At 2 mol % yttrium, this effect is maximized within a stable crystalline framework. However, at the higher 4 mol % content, the excessive incorporating leads to an amorphous structure, which results in a less effective acid site distribution and a decrease in overall acidity. These observations were quantified, and the acid site densities are summarized in Table . The SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst exhibited acid site densities of 3.566 mmol/g and 0.117 mmol/m based on mass and surface area, respectively. Notably, both values were slightly higher compared to the other catalysts.

8.

NH₃-TPD curves of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

The esterification reactions of the precursors decanoic acid, 2-ethylhexanoic acid, and pentaerythritol were conducted using the synthesized catalysts (Figure ) at 220 °C  for 7 h. The conversion rapidly increased in the initial stage and then gradually decreased, following an exponential growth trend. Upon catalyst addition, a significant enhancement in reactivity was observed, especially in the initial reaction rate. While the noncatalytic reaction took nearly 7 h to reach approximately 80% conversion, the optimized SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst achieved this in under 3 h. After the full 7 h of reaction, the final conversion for the SO₄ ^2–/ZrO₂, SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts were 93.7%, 95.5%, and 92.0%, respectively, compared to 81.6% for the noncatalytic reaction. For further comparison, a commercial titanium silicate (TS-1) catalyst was also tested, which yielded a final conversion of 86.8%. This comparison highlights the superior performance of the catalyst synthesized in this study over the conventional TS-1 catalyst for this esterification reaction.

9.

Comparison of conversion versus time for the esterification with (a) no catalyst and with the (b) TS-1, (c) SO₄ ^2–/ZrO₂, (d) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (e) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

Among the synthesized catalysts, while the final conversion values for the SO₄ ^2–/ZrO₂ and SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalysts are comparable, the progression of the reaction over time reveals a crucial distinction that confirms the superiority of the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst. The apparent overlap in final conversion is a natural convergence phenomenon that occurs as the reaction nears completion and the low concentration of remaining reactants, rather than catalyst activity, becomes the rate-limiting factor. The true measure of performance is how quickly this point is reached. Although the SO₄ ^2–/ZrO₂ catalyst showed a slightly higher conversion at 1 h, the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst demonstrated a significantly accelerated and sustained reaction rate thereafter. This superior activity is consistent with its highest measured acid site density (Table ), confirming that the observed difference is a genuine catalyst effect.

From an industrial perspective, this substantial increase in reaction rate is highly significant. It allows for a much shorter process time to achieve high conversion, which translates directly to increased production throughput and reduced energy costs, thereby justifying the use of the catalyst. This trend in reactivity was consistent with the surface acid site densities previously observed. In particular, the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst, which exhibited the highest acid site densities of 3.566 mol/g and 0.117 mmol/m², exhibited the highest conversion among the catalysts tested. Therefore, based on these results, the optimum Y₂O₃ content for achieving the best catalytic activity in this study was determined to be 2 mol %.

To further investigate the intrinsic activity of the acid sites, the turnover frequency (TOF) was calculated. The TOF, defined as the moles of pentaerythritol’s hydroxyl groups converted per mole of acid site per hour, was determined from the initial reaction rates (at 1 h) and the total number of acid site quantified by NH₃-TPD (Table ). The calculated TOF values for the SO₄ ^2–/ZrO₂, SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts were found to be 35.5 h^–1, 29.8 h^–1, and 32.5 h^–1, respectively. Interestingly, the catalyst with the highest number of acid sites (SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02)) did not exhibit the highest intrinsic activity per site; the pure SO₄ ^2–/ZrO₂ catalyst showed the highest TOF. This indicates that the superior overall performance (in terms of final conversion) of the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst is not due to an enhancement in the quality (intrinsic activity) of each acid site, but rather is a direct result of the significantly higher quantity of active sites available for the reaction.

Characteristics of the catalysts following the esterification reaction were analyzed. First, the presence of carbon on the catalyst surface was examined through SEM elemental mapping (Figure ). Unlike the fresh catalysts shown in Figure , the catalysts after the esterification reaction exhibited detectable carbon on their surfaces. Thus, precursor species or reaction products remained adsorbed or bonded to the catalyst surface without being fully desorbed after the reaction. As presented in Table , the carbon content on the catalyst surface after the reaction was slightly lower in the catalyst containing yttrium. This stability is further supported by SEM–EDS mapping images (Figure S1), which display a distribution of Y, S, and Zr on the catalyst surface both before and after use. These results confirm that the catalyst maintained its structural integrity and functioned as a stable heterogeneous catalyst during the reaction.

10.

SEM elemental mapping images of carbon species on the catalysts following the esterification reaction: (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04).

3. Atomic Analysis from SEM-EDS (Atomic %).

	Fresh catalyst					After reaction
Catalyst	Zr	S	Y	O	C	Zr	S	Y	O	C
SO4 2–/ZrO2	65.5	34.5	0	98.6	1.4	63.4	36.6	0	58.3	41.7
SO4 2–/ZrO2(0.98)-Y2O3(0.02)	52.9	38.8	8.3	98.9	1.1	54.5	44.4	1.1	77.2	22.8
SO4 2–/ZrO2(0.96)-Y2O3(0.04)	61	29.7	9.3	98.9	1.1	58.6	39.2	2.2	76.5	24.5

As shown in Figure , new peaks appeared in the FT-IR spectra of the catalysts following esterification. Peaks attributed to C–H stretching vibrations were observed at 2954.7, 2925.8, and 2856.3 cm^–1, while a peak corresponding to CO stretching vibrations was detected at 1739.6 cm^–1. Additionally, peaks resulting from CH₂ and CH₃ bending vibrations were observed at 1548.7 and 1456.1 cm^–1, respectively. A peak attributed to C–O stretching vibrations was also identified at 1053.0 cm^–1. Thus, the esters generated during the reaction remained adsorbed on the catalyst surface, potentially decreasing the catalytic activity. Furthermore, in the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst, additional peaks corresponding to O–H stretching and C–O bending vibrations were observed at 3456.2 and 1365.5 cm^–1, respectively, indicating that a portion of the reactant alcohol was adsorbed on the catalyst surface. ,

11.

FT-IR spectra of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts following the esterification reaction.

To investigate the characteristics of the species adsorbed on the catalyst surface after the reaction, which could lead to deactivation, TPO was performed on the used catalyst. Figure presents the TPO results of the catalysts after the esterification reaction. Two distinct desorption peaks were observed. The peak at <200 °C was attributed to the desorption of moisture and volatile alcohols from the catalyst surface. A strong peak observed between 200 and 350 °C was associated with the desorption of esters generated during the reaction. These results were consistent with the findings shown in Figure . Among the catalysts, the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst exhibited the most intense peak, suggesting that numerous reaction products were adsorbed owing to its higher surface acid density and reactivity. The desorption peak also appeared at a slightly higher temperature compared to catalysts with yttrium, and another peak was observed between 400 and 500 °C, which corresponded to the thermal decomposition of various organic compounds, including amorphous carbon.

12.

TPO curves of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts.

Using the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst, which exhibited the highest conversion, the effects of reaction temperature and catalyst loading on esterification were investigated (Figure ). The reaction temperatures were set at 210, 220, and 230 °C, and the conversion increased with increasing temperature. However, the initial reaction rate significantly increased as the temperature increased. The conversion trends at 220 and 230 °C were almost similar, with final conversions of 95.5% and 93.8%, respectively, after 7 h, with a slightly higher conversion achieved at 220 °C. In addition, the effect of catalyst loading on reaction performance was evaluated at a fixed reaction temperature of 220 °C. After 7 h of reaction, the conversion exponentially increased with increasing catalyst loading, approaching approximately 93.8%. Among the tested conditions, the highest conversion was obtained when the catalyst loading was 2.2 wt %. Notably, using 1.1 wt % catalyst also resulted in a reasonably high conversion of 92.0%, rendering it more economically feasible.

13.

Reaction profiles: (a) effects of reaction temperature and (b) catalyst loading.

3. Conclusions

In this study, the effect of incorporating yttrium into ZrO₂ on the acid site distribution and esterification reactivity of sulfonated catalysts for insulating oil synthesis was investigated. The catalysts were synthesized using a sol–gel method. As the amount of yttrium increased, the crystal growth of the support material ZrO₂ was suppressed, leading to a loss of crystallinity as confirmed by XRD. However, this did not result in a significant change in the catalyst’s specific surface area or primary particle size. Therefore, the observed differences in catalytic performance are not attributed to morphological effects such as particle size, but rather to the changes in the electronic and chemical properties of the catalysts, such as the acid site distribution discussed previously. The binding of SO₄ ^2– to the support was confirmed using FT-IR spectrometry. The TGA results revealed that the decomposition temperature of the SO₄ ^2– ions shifted to higher temperatures after adding yttrium, indicating that the formation of oxygen vacancies within the crystal structure strengthened the interaction with SO₄ ^2–. These structural characteristics resulted in changes in the acid site distribution of the catalyst, with the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst exhibiting the highest acid site density per unit surface area. Esterification reactions were performed using decanoic acid, 2-ethylhexanoic acid, and pentaerythritol as precursors, and all the synthesized catalysts demonstrated excellent reactivity with conversion >90%. Among them, the SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02) catalyst exhibited the highest performance, achieving a conversion of 95.5% after 7 h of reaction. The catalysts developed in this study are expected to replace currently used homogeneous catalysts for synthetic ester insulating oil production, thereby improving catalyst recovery and purification processes. Furthermore, a follow-up study is planned to investigate the long-term stability and reusability of the optimized catalyst, which is essential for confirming its practical viability in industrial processes.

4. Experimental Methods

4.1. Materials

Zirconyl chloride octahydrate (ZrOCl₂·8H₂O, 98%, Sigma-Aldrich, USA), yttrium nitrate hexahydrate (Y­(NO₃)₃·6H₂O, 99.8%, Sigma-Aldrich, USA), ammonium sulfate ((NH₄)₂SO₄), 99%, Sigma-Aldrich, USA), and NH₄OH (98%, Sigma-Aldrich) were used as the precursors for catalyst synthesis. Decanoic acid (capric acid) (C₁₀H₂₀O₂, 98%, Sigma-Aldrich) and 2-EHA (C₈H₁₆O₂, 98%, Sigma-Aldrich) were selected as the fatty acid components for esterification while pentaerythritol (98%, Sigma-Aldrich) was employed as the alcohol. All materials were used as received without further purification. The initial moisture content of the reactants was measured using a Karl Fischer titrator (Metrohm 831 KF) and confirmed to be in the range of 100–500 ppm. This trace amount of water was considered negligible compared to the large amount of water produced during the reaction.

4.2. Synthesis of Catalysts Using the Sol–Gel Method

The catalysts were synthesized using a modified sol–gel method. The final compositions were designed by setting the desired molar ratios of the oxide forms (ZrO₂ and Y₂O₃) and normalizing the total moles of the combined oxide to 1. For instance, the catalyst designated as SO₄ ^–2/ZrO_2(0.98)-Y₂O_3(0.02) corresponds to a target composition of 0.98 mol of ZrO₂ and 0.02 mol of Y₂O₃. Based on this target molar composition, the required amounts of the precursors, ZrOCl₂·8H₂O (0.114, 0.110, and 0.107 mol) and Y­(NO₃)₃·8H₂O (0, 0.0045, and 0.0089 mol), were stoichiometrically calculated and used for the synthesis. The calculated precursor amounts were then added to 500 mL of distilled water under stirring. After the complete dissolution of the precursors, NH₄OH was added dropwise to adjust the pH to 10, resulting in the formation of a white precipitate. The suspension was stirred overnight to ensure uniform dispersion of the precipitate, which was washed and dispersed in 500 mL of distilled water. Subsequently, (NH₄)₂SO₄ (0.5 mol) was added and dissolved under stirring overnight to achieve a homogeneous mixture. This solution was heated to 80 °C to evaporate the solvent, resulting in a white powder that was calcined at 600 °C for 3 h to obtain the final SO₄ ^2–/ZrO₂, SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) powder catalysts.

4.3. Physicochemical Characterization of the Catalysts

The structures of the powder catalysts were determined using X-ray diffraction (XRD; SmartLab, Rigaku, Japan) with Ni-filtered Cu Kα radiation (40.0 kV, 30.0 mA) at 2θ angles of 10°–80°.

The surface morphology of the powder catalysts was examined using scanning electron microscopy (SEM; JEOL JSM IT-800 with BRUKER EDS) operated at an accelerating voltage of 5 kV and in the FlatQuad mode to obtain high resolution surface images. Elemental mapping was conducted using the EDS system in the mapping mode, where distributions of key elements such as Zr, S, and Y were visualized over selected surface regions.

Structural analysis of the powder catalysts was performed using Fourier-transform infrared (FT-IR) spectroscopy (Vertex 80v, Bruker, USA) in the attenuated total reflection mode. To eliminate the effect of moisture, the measurement chamber pressure was maintained at <2 hPa, and 34 scans were recorded in a wavenumber range of 500–4000 cm^–1 with a resolution of 4 cm^–1 for each sample.

The optical properties of the powder catalysts were characterized using an ultraviolet–visible (UV–vis) spectrophotometer (Cary 5000, Agilent, USA). The absorption spectra in a wavelength range of 200–800 nm were recorded by mounting the sample in an integrating sphere.

The surface chemical composition and bonding states of the powder catalysts were elucidated using X-ray photoelectron spectroscopy (XPS) with NEXA G2 (Thermo Scientific) analysis. The XPS measurements were performed using an Al Kα X-ray source with a 400 μm beam size. A flood gun was used for charge compensation, and peak calibration was conducted against the C 1s peak at 284.8 eV for carbon correction.

Thermogravimetric analysis (TGA) was conducted using a TGA analyzer (TA Instruments, TGA Q500, USA) to evaluate the thermal stability and decomposition behavior of the powder catalysts. The measurements were performed under a nitrogen atmosphere (N₂ flow rate: 40 mL/min) to prevent oxidative degradation.

Approximately 5–10 mg of powder catalyst was placed in a platinum crucible and heated from 50 to 800 °C at a constant heating rate of 10 °C/min. The weight loss profile was recorded as a function of temperature, and differential thermal analysis curves were obtained to further interpret thermal events. Temperature-programmed oxidation (TPO) was also performed using the same conditions under an oxygen atmosphere.

Each powder catalyst (0.05 g) was charged in the quartz reactor of a temperature-programmed desorption (TPD) apparatus (Bel Japan Inc., Japan). The catalyst samples were pretreated at 200 °C for 1 h under an He flow (50 mL/min) to remove physisorbed water and impurities, and NH₃ (5 vol % NH₃/He) gas was injected into the reactor for 30 min at 50 mL/min and 50 °C. The physisorbed NH₃ gases were removed by evacuating the samples at 50 °C for 15 min. The furnace temperature was increased from 50 to 600 °C at 10 °C/min under He flow, and the desorbed NH₃ gases were detected using a TCD detector.

N₂ adsorption–desorption isotherms were measured at −196 °C using a BET analyzer (Microtrac BEL, Japan). Prior to analysis, samples were degassed at 150 C for 3  h under vacuum.

Their specific surface areas were calculated using the BET method (P/P_o = 0.05–0.30), and total pore volume was obtained from the adsorption at P/P_o ≈ 0.99 using the BET plot.

4.4. Esterification Reaction

The synthesis of the ester insulating oil was conducted in batch mode in 4 L, 5-necked flask equipped with a mechanical stirrer, heating mantle, thermocouple, and a condenser apparatus (as illustrated in Figure ). The flask was charged with decanoic acid (1.2 mol, 189.9 g), 2-ethylhexanoic acid (0.78 mol, 112.5 g), pentaerythritol (0.4 mol, 54.5 g), and the synthesized catalyst (2.2 wt % of the total reactant mass) at room temperature. The overall reaction is illustrated in Scheme S1. To prevent oxidation, the reaction was conducted under a continuous nitrogen flow. After heating for approximately 1 h to reach the target temperature, the mixture was held at a constant temperature of 220 °C with stirring at 800 rpm. After reaction, the heating mantle was removed, and the mixture was allowed to cool to room temperature. The combination of the high reaction temperature (well above the boiling point of water), the applied vacuum, and the nitrogen stream ensured the continuous vaporization and transport of the produced water to the condenser. The generated water was collected in a Dean–Stark trap after passing through the 500 mm condenser. To measure the amount of water at each monitoring interval, the collected condensate was drained from the trap and washed with a nonpolar solvent (heptane). In this process, any codistilled organic species dissolved into the solvent phase, allowing for the clear separation of the aqueous layer. The volume of this separated water layer was then accurately measured. The collected condensate separated into two layers. Analysis by FT-IR and LC-MS confirmed the bottom aqueous layer contained less than 1% organic impurities. The volume of this layer was therefore considered a reliable measure of the water produced for the conversion calculation, as described in eq :

$$\mathrm{Conversion}(\%)=\frac{\mathrm{Amount\; of}{\mathrm{H}}_2\mathrm{O}\mathrm{actually\; produced}}{\mathrm{Amount\; of}{\mathrm{H}}_2\mathrm{O}\mathrm{theoretically\; produced}}\times 100\%$$ 1

14.

Schematic illustration of the reaction apparatus.

Glossary

Abbreviations

2-EHA

2-ethylhexanoic acid

FT-IR

Fourier-transform infrared

SEM

scanning electron microscopy

TGA

thermogravimetric analysis

TPD

temperature-programmed desorption

TPO

temperature-programmed oxidation

XPS

X-ray photoelectron spectroscopy

XRD

X-ray diffraction

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c06025.

• Esterification of pentaerythritol with decanoic acid and 2-ethylhexanoic acid to form mixed tetraesters (R₁ = n-decyl, R₂ = 2-ethylhexyl); SEM-EDS mapping images of the (a) SO₄ ^2–/ZrO₂, (b) SO₄ ^2–/ZrO_2(0.98)-Y₂O_3(0.02), and (c) SO₄ ^2–/ZrO_2(0.96)-Y₂O_3(0.04) catalysts after the esterification reaction (PDF)

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B.S.K. and E.K. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

This research was supported by the Ministry of Trade, Industry and Energy (MOTIE) under the project (Project Number: 20221A10100011).

Dr. Youngkyun Moon is affiliated with Hanyu SK ETS Co., Ltd. He contributed to the experimental design and the discussion of the results. This collaboration did not involve any financial interests or commercial funding.

The authors declare no competing financial interest.